The present invention is concerned with an anti-locking hydraulic brake system of the type described in U.S. Pat. No. 4,685,747 issued to Belart et al. on Aug. 11, 1987 (corresponding to German DE-OS No. 35 02 451). To prevent the wheels from locking during deceleration, the anti-lock control will begin to operate in the event of an imminent locking danger. For that purpose, the driver or motor is actuated and the brake conduits are blocked so that the brake pedal will remain in its instantaneous position. The pressure in the master cylinder now serves to excite a control valve adjusting the pressure on the outlet of the pump to a value proportional to the pressure in the master brake cylinder. Through actuation of the inlet and outlet valves, pressure fluid can be supplied to and discharged from the wheel brakes so that the pressure in the wheel brakes is adjusted exactly to a value that avoids wheel locking. According to the Belart et. al. patent, the check valve is hydraulically excited so that the regulated pressure is used as a control pressure for switching the check valve into its blocking position.
While this solution is generally satisfactory it is possible during a controlled deceleration for the pressure on the outlet of the pump to decrease rapidly. This can happen, for example, if the wheels are first decelerated on a road section of a low coefficient of friction and are then decelerated on a road section of a high coefficient of friction. When passing from one road section to another, a relatively large amount of fluid pressure is needed in the wheel brakes to adapt to the new friction conditions. If the pump fails to supply this amount of pressure fluid, the pressure on the pump outlet will drop causing the check valve to switch back to its open position. The required amount of pressure fluid will then be discharged from the master brake cylinder so that the volume thereof is quickly depleted if the described change between friction coefficient surfaces, as described, occurs in a rapid sequence. If, in the above-described circumstances, the pressure fluid supply of the pump fails there will no longer be any reserve volume available in the master brake cylinder that would permit an emergency deceleration.